Paper TF-ThP15
Reliability Analysis of Zinc Tin Oxide Thin Film Transistor under Mechanical Stress and NBIS (Negative Biased Illuminated Stress) Condition

Thursday, October 22, 2015, 6:00 pm, Room Hall 3

Amorphous zinc tin oxide (ZTO) attracts attention as a new channel material of switching thin film transistor (TFT). It has remarkable properties like high field effect mobility (>10 cm²/Vs) as well as high uniformity and low processing temperature. In this study, electrical properties like threshold voltage (V_th), mobility, and subthreshold swing of ZTO TFTs with/without mechanical stress were investigated to find the effects of the photo-bias instability of ZTO TFTs on their mechanical stress.

The fabricated ZTO TFTs have a bottom gate and top contact configuration. Thinned Si wafers of 50㎛ thickness were used as substrates to allow ZTO TFTs to be flexible. To measure the effects of mechanical stress to electrical properties and photo-bias instability, the negative biased illuminated stress (NBIS) tests were performed in three illumination conditions with the photo-wavelengths of 400, 450 and 500 nm as well as in two mechanical bending directions, parallel and perpendicular to a channel width of TFTs, with a bending curvature of 40mm.

Under a mechanical strain, a mobility and a subthreshold swing of ZTO TFTs remained unchanged, but a V_th showed noticeable changes. Tensile strain under the bending parallel to a channel width caused more severe photo-instability with more V_th shift. V_th of ZTO TFTs remained unchanged under the 500nm NBIS condition, while for 400 and 450 nm NBIS conditions V_th shifted with a mechanical strain. Particularly, parallel bending of ZTO TFTs under the 400nm NBIS condition showed severe instability, a large and negative V_th shift of –11.6V compared to –7.2V of a non-bended sample. And for the 450 nm NBIS condition, relatively small V_th shift of –6.1V and -4.4 V for bended and non-bended samples, respectively, was observed, showing similar tendency to the 400nm case. But in case of the bending perpendicular to a channel width showed less V_th shift from non-bending condition; -15.8 V to -15.2 V and -5.5 V to -4.0 V for the 400nm and 450 nm NBIS conditions, respectively.

The exact mechanism of this phenomenon is not clear at this point, but the strain in film might change the distance between metal atoms, causing the variation of an orbital overlap in the bandgap and an activation energy for V_th shift. G. Zhang et al. reported that a strain in a channel layer makes difference of ZnO band structure like midgap states from the first principle simulation. The tensile strain can cause more midgap levels in bandgap, and vice versa. It is in a good agreement well with our result. The more reasonable mechanisms of strain to affect the band structure is under investigation.